This invention relates to energy storage systems, and more particularly, to a system and method for balancing the power output of multiple flywheel energy storage systems.
Moderm electrical devices provide many important functions in today""s world. However, these electrical devices are dependent upon a constant supply of electricity in order to perform their marvels. For example, a respirator in a hospital provides important breathing assistance to a patient, but when the electricity fails, it can no longer perform its life-saving function. Hence, it has become important to have a supply of uninterrupted power for critical applications. A reliable source of backup power is frequently called an uninterruptible power supply, or UPS.
One uninterruptible power supply is a battery backup system. In a battery backup system, a number of batteries are connected together to provide power when the main power supply fails. While battery backup can fill a gap in the supply of power to a critical load, it suffers from significant drawbacks. First, the batteries in the power supply are dangerous because they contain highly corrosive acid. Second, the batteries contain heavy metals, such as lead, which pose significant environmental hazards upon disposal. Third, it is not possible accurately to gauge the amount of reserve in a battery, as the power stored in a battery can only be tested under load. It is only when the battery system is in actual use that its power can be measured, and then it is too late to remedy a deficiency if the batteries are run down. Reliability is thus compromised in a battery backup system, as an operator of a critical application cannot know in advance how long a battery backup system will provide power.
As a result of the deficiencies of battery backup systems, flywheel energy storage systems have been developed. In a flywheel energy storage system, a flywheel, turning at a high rpm, drives a motor/generator. The motor/generator generates three-phase electrical current, which is supplied to a rectifier. The rectifier converts the three-phase alternating current from the motor/generator to a constant voltage DC output. The DC power output from the rectifier is supplied to the critical application or load.
Flywheel energy storage systems frequently employ two or more flywheels, each encased in a housing with associated components, in order to provide sufficient power. A multiple flywheel energy storage system may comprise a master-slave arrangement, including a master flywheel energy storage system and one or more slave flywheel energy storage systems, which operate under control of the master flywheel energy storage system. The master unit is connected in parallel with the slave units and to the critical application. Various connections must be supplied between the master unit and the slave units, utilizing complex, unreliable, and expensive circuitry. Of course, a master-slave arrangement is inherently unreliable in that if the master unit fails, the entire flywheel energy storage system is inoperable. However, problems arise in a multiple flywheel system without a master-slave arrangement, because only one of the flywheel units can supply power to the application at any moment in time. Only the flywheel energy storage unit with the highest voltage can supply power to the load. The next highest voltage unit then supplies power when the first unit runs down. Thus, a multiple flywheel energy storage system without a master-slave arrangement cannot supply power to a load that is greater than the output of any single flywheel energy storage unit.
In accord with the present invention, an electrical energy storage system for supplying power to a load comprises a plurality of flywheel energy storage systems, each supplying a power output signal, and a connector circuit. The connector circuit connects the flywheel energy storage systems to the load, but the flywheel energy storage systems are not connected to each other. Each of the flywheel energy storage systems comprises a flywheel turning at an initially predetermined rate, a motor/generator coupled to the flywheel, a bi-directional inverter circuit coupled to the motor/generator and to the load, and a control circuit coupled to the motor/generator and the bi-directional inverter circuit. The control circuit controls the power output signal of the flywheel energy storage system independently of the other flywheel energy storage systems.
Further in accord with the present invention, a method of supplying power to a load comprises the steps of providing a plurality of flywheel energy storage systems, each of which generates a power output signal having a nominal output voltage, calculating the power output of each of the flywheel energy storage systems to generate a calculated power signal associated with each, multiplying each of the calculated power signals by a predetermined amount to generate an associated correction signal, subtracting each of the associated correction signals from each of the nominal output voltages, and reducing the power output signal of each of the flywheel energy storage systems in response to the subtracting step independently of the others.
Also in accord with the present invention, in a backup power supply with at least two flywheel energy storage systems, the flywheel energy storage system comprises a flywheel turning at an initially predetermined rate, and a motor/generator coupled to the flywheel. A bi-directional inverter circuit is coupled to the motor/generator, and a control circuit is coupled to the motor/generator and the bi-directional inverter circuit for controlling the power output signal of the flywheel energy storage system. The control circuit controls the power output signal independently of the other flywheel energy storage systems.
Still further in accord with the present invention, a circuit is provided for balancing the load of a plurality of flywheel energy storage systems. Each of the flywheel energy storage systems supplies a power output signal with a nominal output voltage. The circuit comprises a power output calculation circuit for calculating the power output of each of the power output signals and generating a calculated power signal in response thereto, and a multiplier circuit associated with each of the flywheel energy storage systems for multiplying the calculated power signal by a predetermined amount to generate a correction signal. A subtracting circuit is associated with each of the flywheel energy storage systems for subtracting the correction signal from the nominal output voltage to generate an adjustment signal, and a circuit is associated with each of the flywheel energy storage systems responsive to the adjustment signal for reducing the power output signal of each of the flywheel energy storage systems independently of the others.
Even further in accord with the present invention, in a multiple flywheel energy storage system wherein each flywheel unit supplies a power output signal with a nominal output voltage and has a digital signal processor, a computer program product comprises a computer usable medium having computer readable program code embodied in the medium for causing an application program to execute on the digital signal processor. The computer program code includes a power output calculation program for calculating the power output of an associated one of the flywheel energy storage systems and generating a calculated power signal in response thereto, and a multiplier program for multiplying the calculated power signal by a predetermined amount to generate a correction signal. A subtracting program subtracts the correction signal from the nominal output voltage of the associated one of the flywheel energy storage systems to generate an adjustment signal. A program is responsive to the adjustment signal and reduces the power output signal of the associated one of the flywheel energy storage systems independently of the others.
Still further in accord with the present invention, a method of balancing the load of a plurality of flywheel energy storage systems, each of the flywheel energy storage systems supplying a power output signal having a nominal output voltage and a power output, comprises the steps of calculating the power output of each of the flywheel energy storage systems to generate a calculated power signal associated with each, multiplying each of the calculated power signals from each of the flywheel storage systems by a predetermined amount to generate associated correction signals, subtracting each of the associated correction signals from the nominal output voltages, and reducing the power output of each of the flywheel energy storage systems in response to the subtracting step independently of the others.